862 



Fishery Bulletin 100(4) 



Intercalary 



-^ — Neural arcti 

 , j%4 — Neural canal 

 — Centrum 



A Haemal 

 ' — -'^ arch 



B 



Plane of 

 section 



Figure 1 



(A) Schematic diagram of the section of vertebral column with attached neural 

 and haemal arches from a bluntnose sixgill shark. (B) Neural arch; an initial 

 dorsoventral cut was made perpendicular to the arch face prior to saturation with 

 silver nitrate solution. Subsequent sections (dotted lines) were made to reveal 

 banding patterns. (C) Sectioned neural arch showing band patterns. 



After soaking in silver nitrate, each neural arch was 

 rinsed with distilled water to remove any excess solution. 

 A scalpel was used to cut sections approximately 2-3 mm 

 thick, parallel to the initial cut (Fig. IB). Sections were 

 placed under the white light (400-700 nm) of a dissect- 

 ing microscope for a few minutes to allow bands to appear 

 (Fig. IC). A 2-3 minute soak in 5% sodium thiosulphate 

 (Cailliet et al., 1983) was unsuccessful in halting the de- 

 velopment process and fixing the chemical substitution 

 of calcium salts for silver nitrate. However, a 3-5 minute 

 soak in Kodak Stop Bath SB-la" ( 1.0 L water; 125 niL 287r 

 acetic acid) did halt the development process. The stained 

 neural arch sections were mounted on a microscope slide 

 with an acrylic toluene mounting medium. 



Results 



Of the ten sharks sampled, eight of the sharks were female 

 ( 130-349 cm TL), and two were male ( 140 and 242 cm TL). 

 All sharks were immature. The soak time required for the 

 silver nitrate to penetrate the whole neural arch varied 

 between arches of similarly sized sharks and even between 

 arches from the same shark. Generally arches from sharks 

 less than 200 cm TL, 200-250 cm TL, 250-300 cm TL, 

 and greater than 300 cm TL were removed after 1, 2, 3, 



and 4 hours respectively. It was necessary to use several 

 neural arches per shark in order to differentiate a clear set 

 of bands (annuli). 



The staining occurred only near the outer portion of each 

 arch, and this left the central portion unstained (Figs. 2 and 

 3). Distinct, thin, dark bands were discernible in nine of the 

 10 sets of neural arches examined (e.g. Fig. 2). Bands in the 

 neural arches from the 349 cm female were not all clearly 

 defined even after soaking in the silver nitrate solution for 

 eight hours. In general, bands appeared at regular intervals 

 ( Fig. 2 ). The widths between bands were asymmetrical along 

 the whole edge of the neural arch. These widths were great- 

 er along the top edge compared to widths at the side edges 

 (Fig. 3). In some neural arches, the bands were not continu- 

 ous (Figs. 3 and 4), which made examination of different por- 

 tions of the arch necessary in order to count the number of 

 bands formed. In some arches, bands were identifiable but 

 fainter towards the inner portions (Fig. 4). These bands ap- 

 peared granular, but patterns were still detectable (Fig. 4). 

 The number of bands increased linearly with the total 

 length of the shark (Fig. 5). By using 67.5 cm as the median 

 length at birth (Castro, 1983), the mean estimated growth 

 rate was 25 cm/band (SD=4, «=9). Because all of the speci- 

 mens in our study were immature, a change in the growth 

 rate was not observed, as would be expected when size at 

 maturity was reached. 



